I've been using Marvin Gaye "What's going on?" as a test album a lot recently. The answer is there's actually loads going on! Layers of studio generated depth with different things happening at different depths. The Radford STA25 MkIII I had in did wondrous things with these "layers of detail" that non of my SS amps could quite do but with this in line it all seems to be there with the SS amp!

Very interesting and needs some more investigation